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The outside forces create the non-electrostatic electric field that
cause the positive charges movement from the points with the lower
 potential to the points with the higher potential  .
2 1
Due to the electric field of the outside forces, the electric charges
move inside the battery against the electrostatic forces field at the ends
of the outer circuit, the potential difference is necessary for the direct

current. During this process the outside forces are working. To
characterize the possibility of such work, the electromotive force was
introduced.
The electromotive force is the physical magnitude that

quantitatively equals to the outside force work that is necessary to
transfer the unit of positive charge alone into a closed circuit:

A
  outside (1.1)
q
The SI unit of electromotive force: joule per coulomb (J/C) or
volt (V).

2. The Electromotive Force Generation

Besides voltaic cells, which utilize electrochemical reactions,
other devices that produce chemical EMF are fuel cells, where there is

no electrolyte, but chemicals are introduced directly at electrodes.
Radiant and thermal energy (e.g., a solar cell or a thermocouple) can
also produce EMF. Some other sources of EMF include

thermocouples, thermopiles, and photodiodes.
Dissimilar metals in contact also produce what is known as a
contact electromotive force or contact potential.

The principle of electromagnetic induction states that a time-
dependent magnetic field can produce a circulating electric field. A
time-dependent magnetic field can be produced either by motion of a

magnet relative to a circuit or by motion of a circuit relative to another
circuit (at least one of these must be carrying current), or by changing
the current in a fixed circuit. The effect on the circuit itself, of
changing the current, is known as self-induction; the effect on another

circuit is known as mutual induction.

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